1. Field of the Invention
The invention relates to a multilayer polypropylene film which comprises at least one base layer containing a propylene polymer or a propylene polymer mixture, and resin, and at least one outer layer which comprises a mixture or a blend of two polymer components.
The film according to the invention is distinguished by a characteristic matte surface or appearance and good twist behavior.
The invention furthermore relates to a process for the production of the multilayer film and to the use of the film.
2. Description of Related Art
A particular type full wrapping closure for the purpose of packaging is twist wrapping. This packaging method is particularly known in the full wrapping of relatively small objects. This type of packaging is frequently preferred for round or virtually round articles such as candy, bottles, candles, roll-packed confectionery, bars of chocolate and marzipan, Easter eggs or the like.
The requirements for twist wrapping is that it must not tear at the twist points, but must also not be so rigid that no shrinkage or crumpling occurs during the twisting operation. According to the known art, twist wrappings for candy are made predominantly from cellophane, i.e., regenerated cellulose, un-oriented polypropylene or PVC film, cf. "Verpacken mit Kunststoffen" [Packaging using Plastics] by Gu/ nther Ku/ hne, published 1974, Carl-Hanser-Verlag, Munich, page 63.
Very recently, polypropylene films which have been biaxially oriented to different extents have been proposed for use in twist wrapping.
EP-A-0 479 101 relates to a multilayer, highly transparent, biaxially oriented polypropylene film having very good twist properties which is particularly suitable for twist wrapping. The film has a base layer comprising from 70 to 95% by weight of polypropylene and from 5 to 30% by weight of a low-molecular-weight hydrocarbon resin having a softening point from 130.degree. to 180.degree. C., and the outer layers contain an antiblocking agent having a mean particle size from 2 to 5 .mu.m and possibly polydimethylsiloxane. The outer layers have a maximum thickness of 0.5 .mu.m. The films are furthermore distinguished by good mechanical properties, which are equally good in both the longitudinal and transverse direction. The film has balanced orientation.
EP-A-0 432 452 relates to a transparent twist-wrapping film made from polypropylene which has been monoaxially oriented and has a permanent set of greater than 45% perpendicular to the orientation direction. If desired, the film can be provided with one or more outer layer(s) made from conventional outer-layer materials.
DE-A-35 35 472 describes a film which is also highly suitable for twist wrapping. The addition of siloxane and antiblocking agents to the outer layers gives the film the desired processing properties on automatic, high-speed twist-wrapping machines. The film has the disadvantage of a relatively high resin content, which is necessary for the good twist-wrapping properties. An example given therein is 25% by weight. This high resin content makes the film significantly more expensive.
Known twist-wrapping films have unsatisfactory appearance, processing properties and in particular scratch resistance. Twist-wrapping films having a conventional base layer and a conventional ethylene (C.sub.2)/propylene (C.sub.3)-copolymer outer layer frequently exhibit cloudy structures or optical inhomogeneities.
German Patent Application P 41 35 096.0 discloses multilayer polyolefin films comprising a polypropylene base layer and an outer layer made from an HDPE blend. This HDPE blend comprises HDPE and copolymers and/or terpolymers made from .alpha.-olefins and, if desired, polypropylene. The outer layer of the multilayer film has minimum sheen and maximum haze, giving the film a characteristic matte appearance.
U.S. Pat. No. 4,578,316 discloses a biaxially oriented multilayer polyolefin film whose base layer comprises a polypropylene homopolymer and whose outer layer comprises a blend of polypropylene and MDPE and/or HDPE. This polyolefin film has a low coefficient of friction together with a readily wettable surface. However, this film also has excellent optical transparency.
WO 89/10839 discloses multilayer polyolefin films whose base layer comprises a propylene polymer and whose outer layers comprise either HDPE or an ethylene-propylene copolymer or polypropylene. The film is stretched under particular temperature conditions. It has a low coefficient of friction and good wettability, but simultaneously also has high sheen and low haze.
EP-A-0 367 613 discloses a multilayer film comprising a vacuole-containing polypropylene base layer and an inscribable outer layer which contains a first polymer having a melt flow index of .ltoreq.1 g/10 min and a second polymer which is incompatible with said first polymer. Incompatible being defined as the polymers being present as separate phases. An example given of the first polymer is an HDPE having a density from 0.92 to 0.97 g/cm.sup.3. The incompatible polymer is polypropylene or a copolymer of propylene or a terpolymer of propylene. The film disclosed has an opaque appearance, i.e. it is essentially non-transparent to light. It is disclosed that the film acquires a matte appearance if it is provided with a print. It is additionally preferred to incorporate a filler such as SiO.sub.2 into the outer layer, to improve the inscribability. The SiO.sub.2 -containing outer layer is simultaneously very matte, giving the film a paper-like appearance. The film disclosed has a density of 0.69 g/cm.sup.3. The matte, SiO.sub.2 -containing surface proves to be unsatisfactory on printing. The color is foggy and does not have a uniform appearance. This foggy color is attributed to inhomogeneities which, due to their opacity, do not prominently appear in the film itself, but come out during printing. The printed films are spotty and are not accepted by a film processor.
Films with an HDPE-modified outer layer known from the prior art are unsuitable as twist-wrapping films due to their mechanical properties. In addition, the films have an unsatisfactory appearance.